Thermionic overvoltage protection circuit



April 10, 1951 w RAMBO 2,548,818

THERMIONIC OVERVOLTAGE PROTECTION CIRCUIT Filed Dec. 10, 1945 FlG.| LOWOVERLOAD cmcurr VOLTAGE W IOJ 7/ l 8 5 HIGH TRANSMITTER VOLTAGE 3% F53'ww3vwv m 35 26- SUPPLY \VOLT-AGE CONTROL VOLTAGE INVE/VT'OR WILLIAM R.RAMBO A TTORNE Y Patented Apr. 10, 1951 THERMIONIC OVERVOLTAGEPROTECTION cmourr William R. Rambo, Cambridge, Mass., assignor to theUnited States of America as represented by the Secretary of WarApplication December 10, 1945, Serial No. 634,098

6 Claims.

My invention relates in general to protective devices and moreparticularly to circuits for control of excess voltages and currents inelectronic apparatus.

In many types of electrical equipment it is necessary to guard againstvoltages and currents higher than those normally present in theequipment. This is especially true in circuits employing extremely highvoltages and currents in which a small percentage deviation in the inputenergy may produce a large variation in the voltages present in thesystem. Hence, to avoid arcing, fiashovers, and burnout in circuitcomponents, it is customary to employ automatic apparatus to break highvoltage or high current circuits when there is danger of overload. Thesedevices may take a number of forms ranging from simple circuit breakersto almost completely electronic circuits.

Accordingly, one of the objects of the present invention is to provide aprotective device for electrical apparatus.

Another object is to provide a sensitive and reliable electronicprotective circuit.

Still another object is to provide an electronic protective circuitwhich may be readily constructed with standard components.

These and further objects of my invention will be apparent to thoseskilled in the art upon reference to the following specification,claims,

and to the drawings in which:

Fig. 1 is a blockdiagram showing the application of my invention to atransmitter.

Fig. 2 is a schematic diagram of a preferred embodiment of my invention.

Referring now to Fig. 1, one use of a protective circuit such as myinvention comprises is shown in block diagram form. A high voltagesupply source 5 is connected to a transmitter 6 through a variableresistor l and a switch 8 which form a part of overload circuit 9. A lowvoltage supply provides a power source for overload circuit 9. When thevoltage of source increases to a point where damage is likely to occurin transmitter 6, the voltage across resistor l attains a valuesufficient to cause operation of overload circuit 9 which opens switch8, removing the high voltage from transmitter 6.

The details of an overload circuit constructed according to theprinciples of my invention are shown in Fig. 2, to which reference isnow made. Voltage from a source such as low voltage supply III in Fig. lis applied across a voltage divider network consisting of seriallyconnected resistors l5, l6, and [1. Connection is made from the junctionof resistors I5 and I6 through coil I9 of relay 20 to anode 2| of vacuumtube 22. Screen grid 23 of vacuum tube 22 is connected to the morepositive end of resistor 16 while cathode 24 is connected to the lesspositive end of the same resistor, thereby maintaining a voltagedifferential between screen grid and cathode. Capacitor 26 and resistor21 are connected in parallel between control grid 28 and cathode 24 ofthe tube. Connection is made from cathode 24 through resistor 29, havingcapacitor 30 in parallel therewith, and reset switch 3| to one side ofvariable resistor 32, the other side of which is connected directly tothe negative terminal of the input supply voltage. A glow discharge tubeis connected, in series with a resistor 36, between control grid 28 andthe junction of switch 3| with resistor 32. Capacitor 31 is connected inshunt with the series combination of resistors 32 and 36. Contact 39 ofrelay 2D is connected to the more positive end of resistor I! and arm 40of the relay is connected to the less positive end of the same resistor.Contact 42 is connected between resistors 32 and 36 and arm 43 isconnected to terminal 44 which may be adapted for external connection toa circuit which it is desired to protect. A similar terminal 45 isconnected to the same end of resistor 32 to which capacitor 31 is tied.

Before the circuit is energized, the contacts of relay 20 are open sothat the high voltage circuit of which contact 42 and arm 43 form a partis broken. When the supply voltage is applied to the protective circuit,the voltage appearing across resistor I! of the series voltage dividercauses the neon glow tube 35 to ignite. The current through glow tube 35must flow through resistor 21, thereby setting up a voltage dropsufficient to bias grid 28 of the tube beyond cut-ofi potential withrespect to cathode 24. Therefore, no current flows in coil IQ of relay20 and the contacts of the relay remain open. To apply high voltage tothe protected circuit, reset switch 3| is closed. When this is done, thevoltage which maintains ionization in tube 35 is temporarilyshort-circuited through capacitor 39, which may be several microfaradsin capacity, thus extinguishing tube 35. When the grid bias on the tubeis removed in this fashion, plate current flows in the tube, the initialsurge of current charging capacitor 30 and closing the contacts of relay20. The closing of contacts 39 and 40 short-circuits resistor I! whichinitially supplied ionization voltage for glow tube 3 35 so that thelatter remains nonconducting until there is an abnormal change incircuit conditions. Contacts 42 and A3 close simultaneously withcontacts 39 and 40 thus completing the high voltage circuit.

When the protected circuit draws current from the high voltage supply, asmall voltage drop occurs across resistor 32. This voltage is impressedon glow tube 35 since no current is now flowing in resistors 36 and 21.If for any reason there is an abnormal increase in supply voltage or ashort circuit in the protected device so that the current throughresistor 32 increases beyond a predetermined point, the voltageimpressed on glow tube 35 will be sufiicient to initiate ionizationtherein. Thereupon, tube 22 is again biased beyond cut off allowing thecontacts of relay 2% to open, breaking the high voltage circuit.Ionization voltage for tube 35 is then supplied by the drop acrossresistor ii, maintaining vacuum tube 22 in a nonconductive state untilreset switch 31 is closed. It will be noted that resistor 32 is variable50 that the tripout point may be selected at will.

If an overload should still exist When reset switch 3| is closed,capacitor 38 will be charged by the surge of current through tube 22which also closes relay 20. Contacts 39 and ill shortcircuit resistorll, however the voltage across resistor 32 is sufficient to re-establishionization in glow tube 35. Therefore vacuum tube 22 will again beblocked and the contacts of relay will open. If switch 3| is still heldclosed, conduction cannot occur in tube 22 until the charge on capacitor36 has leaked ofi. This takes place relatively slowly so that normallypressure on switch 3| will be released before relay 29 closes again.Thus my circuit provides protection even during the time in which thereset switch is in the closed position.

In many cases, momentary overloads are not sufilcient to damage theprotected equipment, and hence it is desirable that relay 20 should notbe operated unless an overload persists longer than a predeterminedduration. When momentary surges of voltage occur across resistor 32 theyserve only to charge capacitor 3'! and hence do not provide ionizationvoltage for tube 35. The length of time for which an overload mustendure before the high voltage circuit is broken is determined by thetime constant of resistor 36 and capacitor 31.

Capacitor 30 provides an automatic time delay before which relay 20 maynot be reset after having opened. When switch 3| is closed and capacitor38 is charged by the surge of current through tube 22, the overload mayhave persisted, in which case the relay contacts will immediately openand cannot be reset until capacitor 30 is discharged sufficiently forglow tube 35 to be extinguished. This time delay is chiefly dependentupon the capacity of capacitor 32 and the value of resistor 29.

It has been pointed out hereinbefore that an outstanding advantage of mycircuit lies in the fact that the relay is operated by a sudden currentdifferential rather than a gradual one. Other advantages which have notpreviously been mentioned are: maintenance of the operating accuracy ofthe circuit regardless of changes in tube characteristics caused byageing; the practicability of utilizing a relatively heavy duty relaywhich requires a large current differential for operation; thepossibility of resetting the control relay when the current in theprotected circuit 4 has decreased as little as 1 percent below thetrip-out value.

While there has been described hereinabove what is at present consideredto be a preferred embodiment of the present invention, it will beobvious to those skilled in the art that changes and modifications maybe made therein without exercise of inventive ingenuity. Hence, I claimall such modifications and adaptations as may fall fairly within thespirit and scope of the hereinafter appended claims.

What I claim is:

1. A protective device for electrical apparatus including a normallyconducting thermionic vacuum tube having anode and cathode electrodesand at least one control electrode, a normally energizedelectromechanical relay having circuit switch contacts in a controlvoltage circuit, means for connecting the energizing coil of said relayin the anode-cathode circuit of said vacuum tube for normal energizationand activation of said control voltage thereby, a normallynon-conducting glow-discharge tube, means for connecting saidglow-discharge tube in the control electrodecathode circuit of saidvacuum tube so that said glow-discharge tube when conductive causes saidcontrol electrode to be biassed beyond cut-off potential with respect tosaid cathode, means for developing voltages substantially proportionalto voltages in the electrical apparatus to be protected, means forutilizing the voltages thus developed to effect ionization in saidglow-discharge tube, and means intermediate said developing means andsaid glow tube for delaying the utilization of the developed voltages,thereby deenergizing said electromechanical relay to remove the appliedvoltage from said apparatus to be protected when said voltages increasebeyond a predetermined value only after a given interval.

2. Apparatus in accordance with claim 1 wherein said means fordeveloping voltages substantiall proportional to voltages in theelectrical apparatus to be protected comprise resistive means forming aportion of the circuit for conducting energy to said electricalapparatus to be protected.

3. In a protective circuit for electrical apparatus a thermionic vacuumtube having anode and cathode electrodes and at least one controlelectrode, a gaseous glow-discharge tube, means for connecting saidglow-discharge tube in the con trol electrode-cathode circuit of saidthermionic tube so that when said glow-discharge tube is conductive saidcontrol electrode is biassed negatively with respect to said cathode sothat current flow in said thermionic tube is blocked, voltage developingmeans, means for utilizing the developed voltage to render saidglow-discharge tube conductive, switching means, and means forconnecting said switching means so that when said thermionic tube isconductive said voltage developing means is rendered inactive.

4. In a protective apparatus for limiting the voltage impressed onto autilizing means, thermionic means having anode, cathode and at least onecontrol electrode, voltage divider means connected to the input circuitof said thermionic tube for governing the bias between the cathode ofsaid tube and at least on of the control electrodes, switching meansresponsive to a predetermined current fiow in said tube, said switchingmeans being normally engaged when the voltage to be limited is less thana predetermined value, and means responsive to the voltage to be limitedand influencing the current flow n said thermionic tube, said responsivemeans blocking the current flow in said thermionic tube when the voltageto be limited exceeds a predetermined value whereby said switching meansis actuated substantially simultaneously with the blocking of currentflow through said thermionic tube, and whereby the resistive value of atleast a portion of said voltage divider means is altered when theswitching means is actuated.

5. A device according to claim 1, further including means for supplyinginitial ionization voltage to said glow tube, reset switch means fortemporarily short circuiting said ionization voltage supplying means tothereby establish conductivity through said thermionic tube, and switchmeans controlled by said relay for normally short circuiting saidinitial voltage supplying means.

6. A device according to claim 5, further including a time delay circuitintermediate said reset switch means and said glow tube to permit saidrelay to open if a voltage in said voltage developing means persists.

WILLIAM R. RAMBO.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,297,188 Langmuir Mar. 11, 191915 2,147,781 Ward Feb. 21, 1939 2,153,202 Nichols Apr. 4, 1939 2,250,202Matusita July 22, 1941 2,404,001 Smith July 16, 1946

